Immunological T cell memory, defined as the ability of the host to respond more effectively to a second exposure to antigen, is one of the most important, but least understood properties of the immune system. The cellular basis for T cell memory is thought to be the proliferation of T cells that possess T cell antigen receptors (TCR) specific for the antigen in question, as well as functional improvements in these T cells. Much of what is known about memory T cells is based on tissue culture analysis of T cells separated on the basis of differential expression of certain surface markers, for example, CD45RB. This approach, however, is limited in that it requires disruption of lymphoid tissues such that the anatomy of the immune response is destroyed. In addition, mounting evidence suggests that surface markers do not precisely delineate naive cells, short-lived effector cells, and long-lived memory cells. Therefore, it would be desirable to study T cell memory with a system that allows physical tracking of an antigen-specific T cell population at known times after immunization. We propose to do this using a method where a small number of CD4+ TCR transgenic T cells specific for an ovalbumin peptide are adoptively transferred into otherwise unmanipulated syngeneic normal recipients. The TCR transgenic T cells are not the dominant T cell population in the animal but can be detected in lymphoid tissues by flow cytometry or immunohistochemistry using an anti-clonotypic monoclonal antibody. Memory T cells will be induced in adoptive transfer recipients by immunization with ovalbumin in an artificial adjuvant or with ovalbumin-expressing bacteria. Transgenic T cells will be FACS-sorted from recipient mice at known times after immunization and used to determine whether memory T cells are more sensitive to antigenic stimulation than naive T cells, and if so, whether this correlate- with improved integrin- mediated adhesion and diminished dependence on costimulation. The in vitro and in vivo antigen-stimulated cytokine production potential of naive, effector, and memory T cells produced in vivo will be defined to assess,whether the Th1/Th2 paradigm can be applied to bonafide memory cells. The kinetics and stability of surface molecule changes induced by immunization will be assessed to determine whether they are reliable markers of longlived-memory cells. Information on the timing of changes in adhesion molecules will be used to assess the migration of recently- activated T cells to non-lymphoid tissue, whereas information on the timing of changes in Fas, Fas ligand, and CTLA-4 will be used to assess the appearance of apoptotic cells in the lymphoid tissues during the time that recently-activated T cells disappear. Finally, a pathogenic bacterium expressing ovalbumin will be used to describe the behavior of memory T cells during a protective secondary immune response.